1. Field of the Invention
The present invention relates to an oxygen sensor for detecting the concentration of oxygen present in a gas such as exhaust gas from an internal combustion engine.
2. Description of the Related Art
An oxygen sensor is disclosed in Japanese Patent Publication Laid-Open No. 61-108957. FIG. 1 is originally used to explain the present invention, however since FIG. 1 is suitable to explain the related art, the related art is explained below based on FIG. 1. The oxygen sensor has a sensor portion for detecting the concentration of oxygen and a heater portion for heating the sensor portion to improve its ability to detect oxygen. An example of this oxygen sensor is provided with a sensor portion 10 and a heater portion 20.
The sensor portion 10 comprises a solid electrolytic plate 11 and a vent plate 16 provided with a vent channel 17. The vent plate 16 is mounted on the rear surface side of the electrolytic plate 11. A measuring electrode 12 is mounted on the front surface of the electrolytic plate 11, and a reference electrode 15 is mounted on the rear surface. The heater portion 20 includes an insulating layer 21 mounted on the rear surface of the vent plate 16, together with a heater base 22 having a heating element 25. The vent channel 17 in the vent plate 16 serves to force a reference gas such as air toward the reference electrode 15 on the rear side of the solid electrolytic plate 11.
The solid electrolytic plate 11, the vent plate 16, the insulating layer 21, and the heater base 22 are laminated and bonded integrally. The electrolytic plate 11, the vent plate 16, and the heater base 22 are made of zirconium oxide (ZrO.sub.2). On the other hand, the insulating layer 21 is made of aluminum oxide (Al.sub.2 O.sub.3) to provide electrical insulation between the vent plate 16 and the heating element 25 on the heater base 22.
The insulating layer 21 is made of aluminum oxide, while the vent plate 16 and the heater base 22 respectively located over and under the insulating layer 21 are made of zirconium oxide. Therefore, they are considered each sintered into bulk form (plate form) and then bonded together with an inorganic adhesive.
However, with an inorganic adhesive, there is a problem that the obtained adhesive strength is not sufficient to withstand stress produced by the difference between coefficients of thermal expansion in a cooling-heating cycle under environments in which the sensor is used. In consequence, the vent plate and the heater base will peel off from the adhesive layer.
Accordingly, a sandwich-type oxygen sensor as shown in FIG. 6 has been proposed. This sensor has a heating element 25 which has been previously surrounded by an insulating layer 21 of aluminum oxide. The heating element 25 is sandwiched between a vent plate 16 and a heater base 22. The insulating layer 21 is formed by printing aluminum oxide slurry around the heating element 25 such that the thickness of the aluminum oxide layer is less than 20 .mu.m.
Where printing of the aluminum oxide slurry is utilized, it is difficult to form a compact insulating layer. Also, it is difficult to obtain sufficient electrical insulation at temperatures exceeding 700.degree. C.
To circumvent these difficulties, there is a method of producing a separate heating element and bonding the bottom surface of a sensor portion opposite the electrode of the sensor portion having a hole for introducing the atmosphere with an inorganic adhesive. In this method, however, the adhesive is not reliable at high temperatures. Furthermore, the inorganic adhesive does not form a compact layer, thus deteriorating thermal conductivity.